Ben tote DO fe 
oer ete = 
except several small tu 
E. Billings on the structure of the Crinoidea, ete. 71 
quite sufficient to notice a few of these species to prove that the 
rule laid down by Prof. Wyville Thompson, is not a general rule, 
Fig. 1—This figure is a diagram of the interior of the vault 
of a Crinoid which appears to be Batocrinus icasodactylus 
ville, Indiana, several years ago. The test is in a beautiful 
state of preservation and perfectly empty, so that all of the 
markings on the inner surface can be distinctly seen. There 
are twenty-one arms, nged in five groups (a), and the 
same number of ambulacral openings CP), each just ee 
enough to admit of the entrance of a slen 
(mv) is nearly central, and close to it, on aie ‘posterior de, 
there is a small rudely pense nes space (c) with no markings 
e grooves are scarcely at all 
impressed and, indeed, I think they never are so in any Crinoid, 
except in those which have a thick test. In this specimen 
their course is clearly indicated by the remains of the thin par- 
titions which either separated them or to which the vessels were 
attached. They do not run directly toward the mouth, as 
they would do if that organ were the center of the ambulacral 
system, but to the small space (c) behind it where there ap- 
pears to have been situated a vesicle or some other apparatus, 
to which all of them were united. Whatever may have been 
the structure of this central organ, from which the five main 
grooves radiate, it no doubt represented the annular vessel of 
the recent Echinodermata to which Prof. Thompson alludes. 
Fig. 2—represents the structure of an Amphoracrinus from 
the Carboniferous rocks of Ireland,—precise locality and spe- 
cies not determined. There are ten arms; the test is very 
thick; the ambulacral channels converge to the central point (¢) 
but do not quite reach it ; the mouth (mv) is about half way 
between the center and the margin. In this Crinoid it is per- 
fectly impossible that the mouth can be the center of the radial 
system because the two anterior passages, between which it is 
situated, are for their whole length tunneled, as it were, 
